First, a MIMO technology applied to the present invention will be briefly described.
The MIMO is abbreviated from the term “multi-input multi-output” and indicates a method of employing multiple transmission antennas and multiple reception antennas so as to improve transmission/reception data efficiency, instead of a conventional method using one transmission antenna and one reception antenna. That is, the MIMO technology indicates a technology of collecting data pieces received via several antennas and completing one message, not depending wholly on a single antenna path in order to receive one message. According to the MIMO technology, a data transmission rate is improved in a specific range or a system range can be increased with respect to a specific data transmission rate. That is, the MIMO technology is the next-generation mobile communication technology which is widely applicable to mobile communication terminals and repeaters.
FIG. 1 is a view showing the configuration of a general MIMO antenna system.
As shown in FIG. 1, if the number of antennas is increased in both a transmitter and a receiver, a theoretical channel transmission capacity is increased in proportion to the number of antennas, unlike the case where multiple antennas are used in only one of the transmitter and the receiver. Thus, frequency efficiency can be remarkably improved.
After the theoretical capacity increase of the MIMO system was proved in the mid-1990s, various technologies for substantially improving a data transmission rate have been actively developed up to now. Among them, several technologies are already applied to the various radio communication standards such as the third-generation mobile communication and the next-generation wireless local area network (LAN).
According to the researches into the MIMO antenna up to now, various researches such as researches into information theory related to the computation of the communication capacity of a MIMO antenna in various channel environments and multiple access environments, researches into the model and the measurement of the radio channels of the MIMO system, and researches into space-time signal processing technologies of improving transmission reliability and transmission rate have been actively conducted.
The MIMO technology includes a spatial diversity method for increasing transmission reliability using symbols passing through various channel paths and a spatial multiplexing method for improving a transmission rate by simultaneously transmitting a plurality of data symbols using a plurality of transmission antennas. Recently, researches into a method of obtaining the respective advantages of the above two methods by combining the two above-described methods are ongoing.
Hereinafter, a method of using an Alamouti code for transmission diversity in the MIMO system will be described.
For the easy analysis in a transmission diversity (hereinafter, abbreviated to “Tx Div”) structure using the Alamouti code, one can change the concept of the receiver as an equivalent receiver (Rx) shown in FIG. 2
FIG. 2 is a view showing the structure of the equivalent receiver in order to facilitate the description of the system using the Alamouti code.
That is, if a conjugate complex number is applied to a second reception signal for mathematical modeling of the reception signal in the diversity transmission method using the Alamouti code, more efficient mathematical modeling is possible. FIG. 2 shows that the reception signal can be expressed in a matrix form by applying the conjugate complex number to the receiver at a time 2 or a frequency 2. Hereinafter, the detailed mathematical modeling using the conjugate complex number will be described.
As the matrix of the Alamouti code, two matrixes can be considered as follows.
                    [                                                            s                1                                                                    -                                  s                  2                  *                                                                                                        s                2                                                                    s                1                *                                                    ]                            Equation        ⁢                                  ⁢        1                                [                                                            s                1                                                                    s                2                                                                                        -                                  s                  2                  *                                                                                    s                1                *                                                    ]                            Equation        ⁢                                  ⁢        2            
In the matrixes of Equation 1 and Equation 2, each column denotes a time or a frequency and each row denotes the antenna. In more detail, the matrix of Equation 1 indicates a general equation of a matrix which is first suggested in the paper of Alamouti, and Equation 2 indicates a matrix used in the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) standard. That is, Equation 2 indicates a matrix which is reconfigured such that the signal transmitted via an antenna 1 by a single-input single-output (SISO) scheme using only one antenna becomes equal to the signal transmitted via the antenna 1 by an Alamouti scheme.
In the above-described equations, if each column denotes the time, the Alamouti code is used as a kind of a space time block code (STBC) and, if each column denotes the frequency, the Alamouti code is used as a kind of a space frequency block code (SFBC).
In the current discussion about the next-generation radio communication system, a method of spreading transmission signals by orthogonal codes and transmitting the spread signals via multiple antennas according to the Alamouti scheme is used. In this case, an interference imbalance problem may be generated between specific codes, but has not been considered seriously up to now.